Methods and apparatuses for joining sheet formed members together, thereby producing leakproof or non-leakproof joints, are previously known. Of particular interest in some applications is a type of leakproof joint which is made by means of drawing said sheet formed members into a cup-shaped or protruding portion having a cylindrical or slightly conical side wall and a bottom wall and subsequently compressing said bottom wall creating a lateral extrusion of the same thereby forming a laterally enlarged shape which mechanically interlocks the sheet formed members. The present invention, however, is also concerning other types of joints.
U.S. Pat. No. 4,459,735 discloses an apparatus, a method and a joint of this type. The method is of the single stroke type which means that the whole procedure takes place during one single relative movement between a punch and a coacting die. For the compression of the bottom wall of the cup-shaped portion an anvil is arranged fixed at the bottom of the die cavity which cavity is laterally expandable.
Double-stroke methods are also known from e.g. WO 89/07020 according to which the compression takes place during a second stroke outside the die cavity. This cavity is generally laterally non-expandable.
One problem with the leakproof joints of the above type is the relatively low resistance against so called shear and peeling forces.
It turns out that the drawing depth into the die cavity and the gap between the punch and the die cavity are critical parameters.
If the drawing depth is too big and/or the gap between the punch and the die cavity is too small the side wall of the cup-formed or protruding portion, especially on the sheet formed member touching the punch, will be too thin and there is a risk that this side wall will break when exposed to forces tending to separate the members. The problem is emphasized when the joint is made between more than two sheets.
On the other hand, if the drawing depth is too small and/or the gap between the punch and the die cavity is too big, the lateral extrusion of the bottom wall during the compression will not be sufficient to create the interlocking between the sheet formed members and there is a risk that the members separate for that reason, when exposed to forces.